For the manufacture of cigarettes and other tobacco products it is necessary that the relatively dense and rigid stem of the tobacco leaf be separated from the remainder of the leaf, which is known as the "strip" or "lamina." While the stems can be incorporated in tobacco products, because of their rigid character they must go through a separate set of processing operations before they can be blended back with the lamina. Not all of the stem must be removed, only what is defined as "objectionable stem" having a diameter in excess of a dimension specified by the manufacturer.
To separate objectionable stems from the lamina, a process known as threshing is used, in which the leaves are torn or shredded by rotating fingers or vanes into progressively smaller pieces. In the process, lamina portions tend to be dislodged from stem portions. The threshed material is then "classified" or separated into a lighter fraction, which comprises a higher lamina content than the feed, and a heavier fraction, comprising higher proportion of stem than the feed. Such separation is typically carried out as an air separation wherein an air current lifts and carries off the lighter pieces from the heavier pieces. Typically several threshing stages are employed; by repeatedly threshing the heavy or stem rich portions to finer pieces, remaining portions of lamina attached to the stems ("flags") are dislodged and separated as lighter fractions, until virtually all the lamina has been separated. Often five, and sometimes as many as seven or even more progressively finer threshing stages may be used, with a separation after each stage.
The particular manner in which tobacco is threshed depends on the type of "package" in which it is delivered by the grower. In general, three different types of "packages" are in common use. Some tobacco, especially flue cured tobacco, is sold in "sheets," wherein individual leaves, stripped from the stalk, are placed loosely in an unoriented and random manner in a large burlap sheet and the corners tied together. These sheets of leaves are dumped directly into the threshing process without attempting to orient the leaves; the cost of orienting leaves would be excessive. This is called "whole threshing"; it requires more threshing capacity, because the entire leaf must be threshed. Whole threshing produces relatively more fines.
Tobacco is also sold in the form of "hands," in which the leaves are arranged with their stem ends together, tied with another leaf. The leaves in a hand are usually of somewhat different lengths, so that while the stems are together at the butt, the leaf tips are not all at the same position. Tobacco in hand form can be whole threshed, as with sheets; or it can be tipped and threshed. In the latter process each hand is manually placed on a conveyor, with the end opposite from the butt aligned against a guide rail. So aligned, the hands are conveyed past rotating cutters which cut off the outer 8" to 12" of the hands. The cut-off portions average roughly 10% by weight of the hands; because they have almost no stem content, they are not threshed. The remaining approximately 90% of the hands must however be threshed. Tipping and threshing hand tobacco thus requires a little less threshing capacity than whole threshing, but is more labor intensive because of the manual effort required to properly position the hands on the conveyor.
Recently, tobacco as increasingly being marketed in bale form, a third type of "package." In baling, the individual leaves are arranged in a bale-forming box so that their stems are at the respective ends of the box, with the tips pointing toward the opposite ends of the box. However, the tips are not necessarily at the center of the bale: the tip of a long leaf (for example 24" to 30" long) will extend past the center and may approach the opposite end, whereas the tip of a short leaf (less than 18" long) will not extend to the center. When short leaves are being baled, some leaves may be placed centrally in the forming box, to act as bridges between the leaves on each side of center. The bale is compacted under pressure and is tied transversely. Bailing is further described in "Packaging and Handling Burley Tobacco in Bales at the Farm," by Duncan and Smiley, University of Kentucky College of Agriculture, publication ID-39, to which reference may be had.
Baled tobacco can either be whole threshed, or tipped and threshed. Whole threshing capacity requirements for bales are similar to those for sheeted tabacco. Bales to be tipped and threshed are separated into longitudinal layers (roughly 2" to 4" thick), and then these layers are pulled apart endwise to form two rough halves. However, pulling bale layers into halves is a laborious job; the layers, compacted under pressure, are not easily separated transversely. Moreover, additional fines are produced when layers are pulled apart. Once separated, each half layer is then placed on a conveyor with its "non-stem" end adjacent a guide rail. That end is cut off and the remainder of the half layer is threshed as previously described. The cut off ends (roughly 10% of the weight of the bale) can be processed without threshing, but a very large proportion of the bale still must be threshed, and again, a relatively large investment in threshing capacity is required to handle a given number of bales per unit time.